Modern communication systems that operate near theoretical limits to meet the ever increasing demand for high speed, reliable data transmission, employ equalization techniques in transmitters, receivers or both to optimize or nearly optimize transmission and reception. Such equalization is done digitally by adaptive digital filters in order to provide a flexible way of accommodating different types of channels as well as different types of noise environments. In digital subscriber line (xDSL) environments, the transmitted signals suffer from a number of impairments including crosstalk, attenuation and interference caused on one twisted pair due to signals that leaks from another twisted pair, intersymbol interference (ISI) due to line attenuation and delay variations with frequency which causes successive transmitted symbols to interferer, thereby, causing the symbol-by-symbol detection at the receiver to be inadequate and unreliable. Today's communication systems often rely on the rapid transmission of successive signals to represent a selected sequence of information-bearing symbols. Typically, ISI manifests itself as secondary signal components that hamper the detection of the primary, information-bearing signal components.
In one model, the non-ideal channel may be characterized as having a number of signal paths of different lengths coupling a common transmitter and receiver. For this characterization, the secondary signal components resulting from ISI may be thought of as “echoes” of the signals that occur during transmission over the multiple propagation paths. Because of the presence of the aforementioned ISI, the use of equalizers are common in the industry. Equalizers typically cancel the secondary signal components or constructively combine the secondary signal components with the primary signal components to improve reconstruction of the intended symbols.
Some equalization techniques used in existing receivers may employ a decision feedback equalizer similar to the one shown in FIG. 1. In FIG. 1, the signal is transmitted from transmitter 105, through a channel 110, where the signal may become corrupted from intersymbol interference (ISI) caused by the spreading of a transmitted symbol. This in turn may interfere with the immediately adjacent transmitted symbol and, in some severe cases, with other symbols in the data stream. Noise, mostly white noise 120, is added to the corrupted transmitted signal in summer 115 and passed on to receiver 160 for processing. The signal is sampled at the input of the receiver 125 at T/U rate, T denoting the symbol time and U being an integer. The sampling time is typically lower than the Nyquist sampling time. The feedforward filter 130, adjusts the phase of the 131, so that it appears that the 131 is caused by previously transmitted symbols (causal ISI). Since the previously transmitted symbols are available in the receiver 160, the feedback filter 150 receives at its input the actual signal nearest to the estimated outputted by a decision making device such as a slicer 145. This is then subtracted from the feedforward filter 130 output at the summing device 140.
In existing devices, the oversampling rate U, is mandated in order to achieve optimal performance of the DFE. However, this mandated value requires the feedforward filter 130 to have U times as many taps as the number of symbols should cover. This increases resource (memory and calculation speed) requirements for the receiver 160. Other drawbacks of current systems also exist.